A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
A typical fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrolyte-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of generating a quantity of electricity sufficient to power a vehicle.
During periods of non-operation, a quantity of air accumulates in the anodes of the fuel cell stack. Upon start-up of the fuel cell stack, hydrogen is supplied to the anodes. The hydrogen contacts the air and creates a “hydrogen-air front” that passes over the anodes. The hydrogen-air front is known to degrade fuel cell performance. In particular, the presence of both hydrogen and air on the anode results in a localized short between a portion of the anode that sees hydrogen and a portion of the anode that sees air. The localized short causes a reversal of current flow and increases the cathode interfacial potential, resulting in a rapid corrosion of the fuel cell carbon substrates and catalyst supports. The rate of carbon corrosion has been found to be proportional to a time that the hydrogen-air front exists and a magnitude of the localized voltage at the hydrogen-air front.
To avoid damage to the fuel cell system, the fuel cell system can be shutdown using a hydrogen-hydrogen shutdown strategy, wherein hydrogen is present on both electrodes (anode and cathode) after the shutdown procedure is complete. If the fuel cell system is restarted before air leaks into the cathode no damaging carbon corrosion will occur. However, if air leaks into the cathode while hydrogen is still present, a stagnant hydrogen-air front can form on the anode which will cause carbon corrosion on the cathode electrode.
Additionally, a freeze event may occur while the anode compartment is full of stagnant gas (hydrogen, air, water vapor in some combination). As the temperature drops, the stagnant gas begins to condense throughout the anode compartment. Unfortunately, the gas may condense on valves and other critical moving parts, and then freeze. The frozen condensation can render the valve (or other critical component) inoperative during startup, thereby leading to increased damage to cells upon startup and/or a failed freeze start.
It would be desirable to provide a fuel cell system and a method for minimizing carbon corrosion in the fuel cell system, wherein the system and the method maximize the durability of the fuel cell system while not sacrificing a hydrogen-hydrogen shutdown strategy.